A hollow door, as the name implies, is a door that is not made of solid wood or other material. Since there is little material to screw or nail into, securing items to a hollow door requires a special anchor. Choosing the right kind of anchor will offer strength and support.
Expansion Anchors and Hollow Wall Anchors
While there are a few subtypes, anchors can be divided into two main classes or types: expansion and hollow wall. Expansion anchors "expand" as a screw is inserted into them. They are used in solid structures, from wood and masonry to drywall. The important thing to note about expansion anchors is they are only as strong as the material they are installed in. This means that soft material will offer less support, no matter how strong the expansion anchor is.
The other type of anchor, the hollow wall or hollow door anchor, is used in material that is thin or hollow. It spreads within the opening or the hollow to provide support. Once the anchor spreads open, it cannot be pulled back through the opening.
Anchor Subtypes - The Toggle Bolt
Two subtypes comprise the hollow-door anchor family: toggle and molly. Toggle bolts have spring-loaded wings attached to them. They are inserted by drilling a hole large enough to accommodate the bolt with its wings folded or closed. Once the bolt passes through the opening to the hollow space inside, the wings open. As the bolt is screwed, the wings tighten against the back of the hollow space, holding the anchor in place.
A toggle bolt can hold a good amount of weight, depending on the size of the bolt and spread of the wings. The larger bolts and greater wing spreads will support more weight. Toggle bolts do, however, require a larger hole be drilled. And upon removal of the bolt, the wings are lost, as they fall to the bottom of the hollow door or wall. This means that a new toggle bolt has to be inserted. While this may be considered a negative, the toggle bolt is also the easiest hollow door anchor to remove.
Anchor Subtypes - The Molly Bolt
The molly bolt is another type of hollow door anchor. It is a bolt surrounded by a metal sleeve. This is why it is also known as a sleeve anchor. Once the anchor is inserted into the hole, the bolt is turned. As it turns, the metal sleeve starts to expand. It presses against the inside of the wall or door, thereby securing the anchor. A molly setter is a device that can expand the anchors of a molly bolt without using a screwdriver.
The nice thing about molly bolts is they typically require smaller drill holes. They are also able to have the bolt removed and reinserted with harm to the anchor. Molly bolts are available in various thicknesses. And "drive mollies" have a pointed tip that allows them to be driven in with a hammer.